Carburetors for multi-purpose engines supply a considerably lower quantity of fuel to the engine in comparison with carburetors that supply fuel to four-stroke engines, such as automobile engines. Significant changes in fuel mixture ratio result from inaccuracies in carburetor component placement and dimension. Differences in engine performance must also be taken into consideration. All of these factors make it necessary to be able to adjust carburetor fuel flow quantity separately for each individual engine.
Given this necessity, a manually adjustable fuel valve is included in the design of some carburetors. Such valves comprise a needle-shaped, tapered valve that remains inserted into the fuel jet and is mounted on the end of a threaded rod that has an extension at the opposite end. The extension protrudes from the carburetor body into which the threaded rod is screwed. By twisting the extension, the needle valve can be moved back and forth within the carburetor body, thus changing the effective cross-sectional area of the jet. This adjusts the quantity of fuel flow through the jet. Both the main fuel jet and the low-speed fuel jet can be equipped with such valves, thus making it possible to adjust fuel flow quantity separately for each jet. In order to obtain the appropriate quantity of fuel flow, these valves are normally adjusted by the manufacturers of the carburetors and engines, and by the manufacturers of the vehicles or the appliances in which the carburetors are used. However, in certain situations, the user of the engine will make adjustments in an attempt to maintain performance in different locations and under different operating conditions or to improve performance in cases of temporary loss of engine performance. As a result, an excessively rich or excessively lean fuel and air mixture is created, often resulting in less engine power, worsening of the quality of the exhaust, engine stalling, and other engine troubles.
An additional issue to consider is that regulations governing the emissions of multi-purpose engines, which have been put into effect in recent years, make it necessary to equip these engines with a limiting device that allows the user to make adjustments, after the manufacturer has adjusted the carburetor valves, substantially only within the range allowed by law. These devices must also be constructed such that they are difficult to remove from the carburetors.
Devices to limit the adjustment of the fuel adjustment valve have been described in the art. U.S. Pat. No. 3,618,906 describes a cap that has been installed on the end of the adjustment valve. The cap is equipped with a radially protruding appendage that limits adjustment to within one revolution because the appendage is obstructed by the carburetor body acting as a stopper. U.S. Pat. No. 5,236,634 describes valves for both the main fuel jet and the low-speed fuel jet as being placed parallel and adjacent to each other and having a cap with an appendage being obstructed by the other adjustment valve, or its extension acting as a stopper.
However, both of these valve adjustment limitation devices protrude from the carburetor body. Their exposure makes it easier for the user to remove them with a bit of ingenuity. Thus, these devices do not prevent deliberate and resolute tampering by the user.
Other shortcomings with these designs exist during the manufacturing process. Either the valves have to be assembled provisionally so as not to slip out prior to adjustment and, after adjustment of the valves, the cap is installed permanently in a position where its appendage is in contact with the stopper, or the valves are installed only after adjustment with the appendage of the cap in a position in contact with the stopper, without provisional assembly. Not only is it difficult to assemble the very small parts one by one, by hand, but in some cases the appendages arc not positioned correctly in relation to their stoppers. This results in some carburetors having a wider adjustable range in one direction, which could possibly produce an excessively rich or excessively lean mixture and make it substantially possible to operate outside the legal limit for emissions.
Therefore, it would be desirable to have a limiting device for a carburetor, having manually adjustable valves placed parallel and adjacent to each other and that are able to adjust the effective cross-sectional area of the main and low-speed fuel jets separately, being capable of preventing deliberate and resolute tampering by the user, eliminating the difficulty in handling small parts, and preventing the emissions, when the engine is being used in a normal manner, from exceeding the legal limitations due to an inaccurate setting made by the manufacturer.
A still further issue to consider relates to the manner in which adjustment valves of the prior art arc fixed in a prescribed adjustment position. Ordinarily, a compression coil spring is mounted around the threaded rod between the main body of the carburetor and the head portion in order to fix the adjustment valve in a prescribed adjustment position. However, since there is a slight gap between the female threads formed in the screw hole of the main body of the carburetor and the male threads formed on the threaded rod, the following problem arises: when the threaded rod is screwed into the prescribed adjustment position while being pressed with a screwdriver which is engaged with the head portion, and the screwdriver is then released, the compression spring causes the adjustment valve to return in the axial direction by an amount corresponding to the gap between the aforementioned male and female threads. As a result, the flow rate is thrown out of adjustment, which may have a serious effect on the air/fuel ratio, especially in the carburetor of a multi-purpose engine. Furthermore, since the adjustment valve is arranged so that rotation of the valve is prevented by contact friction between the compression spring and the head portion of the threaded rod, it is necessary to use a fairly long spring, and to cause the spring to contact the head portion with a strong force in order to prevent rotation of the adjustment valve. As a result, the threaded rod and head portion protrudes by a considerable amount from the main body of the carburetor. In cases where the carburetor is enclosed in a housing and attached to a multi-purpose engine, the size of the housing must therefore be increased. Furthermore, since the protruding parts are long, the rotational moment generated as a result of vibration of the engine or vibration of the machine or vehicle, etc., is large, so that the adjustment valve may rotate, thus causing the air/fuel ratio to be thrown out of adjustment.
Furthermore, it has been suggested to use two adjustment valves in a carburetor for a multi-purpose engine, i.e., one for the main fuel feed and one for the low-speed fuel feed. (See, for example, Japanese Utility Model Application Kokai No. Sho 61-134555.) In such a circumstance, the two adjustment valves are installed parallel to each other and in close proximity to each other. As a result, there may be contact interference between the respective compression springs, so that the rotation-stopping function is lost.
To address this problem, Japanese Patent Application Kokoku No. Hei 1-28220 proposes an arrangement in which a square retaining plate made of an elastic synthetic resin is used to prevent rotation instead of the compression coil spring. The retaining plate is provided with a hole having a diameter slightly smaller than that of the threaded rod, and the threaded rod passes through the hole while cutting threads in the edge of the hole as it is screwed into the screw hole in the main body of the carburetor. Specifically, a thin square recess is formed in the main body of the carburetor, overlapping the screw hole of the main body, and the square retaining plate is inserted into this recess. The threaded rod passes through the retaining plate while being screwed into the carburetor screw hole. Since the threads of the threaded rod are engaged with the threads cut in the edge of the hole of the retaining plate, both rotational movement and back-and-forth movement in the axial direction of the threaded rod are prevented by the back surface and edge surfaces of the retaining plate contacting the facing inside surfaces of the recess. In this structure, a recess for inserting the synthetic resin plate must be formed in the main body of the carburetor, requiring extra steps in the manufacture of the carburetor. In addition, the retaining plate must be inserted into the recess so that the hole in the retaining plate is concentric with the screw hole. As a result, such a technique presents a number of disadvantages.
Therefore, it would be desirable to have an easy to assemble fuel adjusting device for a carburetor, having manually adjustable valves placed parallel and adjacent to each other and that are able to adjust the effective cross-sectional area of the main and low-speed fuel jets separately, being capable of preventing rotation of the adjustment valves, and eliminating the problems of return of the adjustment valves after adjustment of the valves with a screwdriver.